Idiom: Achilles's heel

A person’s weakness or the vulnerable spot in his
or her character. #ELT #ESL #learnenglish
Source: http://americanenglish.state.gov/resources/loop
Also: https://en.wikipedia.org/wiki/Achilles%27_heel
http://www.wordfocus.com/achilles-heel-story.html
http://www.englishblog.com/2011/09/cartoon-achilles-heel.html